Power amplifiers are used to amplify electrical signals in a wide variety of applications including cellular telephony, and radio and television broadcasting. For power ranges of up to about 100 watts, the most common power device used in these amplifiers is a bipolar junction transistor (BJT). In many cases, the power device is not a single transistor, but is composed of several transistors connected in a parallel circuit arrangement. This parallel circuit arrangement creates some technical problems for circuit designers since a limited number of weak transistors are responsible for circuit failure. Two of these problems, thermal runaway and over-voltage breakdown, are further explained below.
Thermal runaway: In an amplifier that is made up of several transistors connected in parallel, it is desirable for all the transistors to share the bias current equally. However, when bipolar junction transistors are used, the transistors with the highest temperature tend to carry more current. This can lead to a condition called thermal runaway. It a transistor carries a little more current than the others in the circuit, it will also dissipate more power, which will tend to heat the transistor even more. Since the transistor is now even hotter, it will tend to carry even more current. This self-heating operation can escalate until an over-current condition is reached and the transistor and amplifier circuits fail.
The most common solutions to prevent thermal runaway are: 1. To connect a ballast resistor in series with each of the transistors in the array; 2. To use a fuse to protect the entire circuit from high current conditions. This is, unfortunately, not an optimal solution since it requires field replacement (i.e. substantial costs). Also, for some integrated circuit technologies, a fuse would not protect the amplifier from damage during thermal runaway due to current hogging effects; and 3. To use thermal shutdown circuitry. A disadvantage of this solution is that it would require a reset signal and possibly field maintenance (i.e. substantial costs).
Over-voltage breakdown: Any transistor has a maximum operating voltage, above which the device will cease to function properly and may be damaged, Typically, when the applied voltage exceeds the maximum operating voltage, the transistor enters an operating state where the device current is uncontrolled and extremely high. This can lead to simultaneous over-voltage and over-current conditions. In an integrated circuit, which contains several transistors, the maximum operating voltage of the individual devices will not be uniform. Instead, they will have an operating range. In a mature manufacturing process, this voltage range will be relatively narrow and easily described by a statistical variation. On the other hand, in new or leading edge manufacturing processes, the voltage range will be relatively large due to material and process defects. The problem that this creates is that the circuit's operation is limited by the weakest transistor.
One solution for avoiding over-voltage breakdown is to limit the voltage that is applied to the circuit to a value that is lower than the maximum operating voltage for the weakest transistors in the circuit. For example, a circuit optimally designed to operate at 7 volts will be operated in the 5 volts range. Having to work in a lower design voltage range requires a larger number of power transistors (e.g. 20% to 25% more) to obtain the same power gain. The direct result in Lhis case is an increase in size of the amplifier circuit, hence an increase in costs.
When considering the above background information, it is clear that there is a need for a device which will permit continuous operation of transistor inclusive power devices for longer periods, thereby avoiding unnecessary field maintenance costs. Furthermore, this device should take up less space than those performing similar functions in existing integrated circuits.